1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system. More particularly, the present invention relates to a charging unit that charges a photosensitive member with electricity.
2. Description of the Related Art
Conventionally, in an image forming apparatus that use an electrophotographic process, a charging unit is used to charge the surface of an image-bearing member (photosensitive member). As the charging unit, a noncontact charging unit, such as a scorotron charging unit or a corotron charging unit, is used. In addition, in consideration of the environment, a contact charging unit is also used which has a charging member, such as a charging roller or a charging brush, that is in contact with a photosensitive member, to charge the photosensitive drum. By using the contact charging unit, it is possible to reduce the amount of ozone emission.
For example, in the situation where a charging roller is used as the contact charging unit, an electric charge is directly applied to the surface of the photosensitive member by applying a direct-current voltage of about 1 kilovolt to the charging roller that is in contact with the surface of the photosensitive member thereby charging the surface of the photosensitive member to a predetermined potential. However, because the surface of the charging roller, to which the voltage is applied, and the surface of the photosensitive member with which the charging roller is in contact have microscopic irregularities, spot-like charging unevenness can occur on the surface of the photosensitive member.
As a method for preventing such charging unevenness it is known, for example, to uniformly charge the target surface by applying a pulsating-current voltage having a peak-to-peak voltage that is at least twice the charge-starting voltage to an electrically conductive member (charging member). This forms an oscillating electric field between the target member and the electrically conductive member.
As a method for developing an electrostatic latent image formed on the photosensitive member, a so-called jumping development method is sometimes used. This method can develop the electrostatic latent image on the photosensitive member into a visible image by forming a uniform thin developer (toner) layer on a developing roller that functions as a toner bearing member, which is close to the surface of the electrostatic latent image, and applies a developing bias. A direct-current bias and an alternate-current bias are superposed to the developing roller to thereby electrically attract the toner causing the toner to attach to the electrostatic latent image. This improves the stability of the density of the image and reduces fogging.
In such a jumping development method, where a superposed direct-current bias and alternate-current bias is used for both the charging bias and the developing bias, problems with the unevenness of formed images can occur. This can be caused by the frequency of the alternating-current component of the charging being similar to: the frequency of the developing bias; frequency of an integral multiple thereof; or the frequency that is an integral submultiple thereof.
A possible cause of the problems is that when the alternate-current components of the charging bias and the developing bias are applied at a close range, at the same time, electromagnetic induction or the like occurs in the wires of the high-voltage supply board, the charging unit, and the developing unit. Therefore, the alternate-current component of the developing bias may be (electromagnetically) induced into a charging bias. Likewise, the alternating-current component of the charging bias may be (electromagnetically) induced into the developing bias.
In particular, when the alternating-current component of the developing bias is induced into the charging bias, beats are generated between the original frequency of the alternating-current component of the charging bias itself and the frequency of the induced alternating-current component. If these beats cause a charging unevenness during the formation of the electrostatic latent image, the formed image can look jittery, blurry.
Therefore, a method for preventing interference between the charging bias and the developing bias is proposed. For example, a method (hereinafter referred to as “method A”) for preventing the occurrence of radiant (electrostatic) noise to avoid surrounding electronic devices from malfunctioning is known. This is accomplished by surrounding a contact nipping portion formed between the charging member and the target member with a shield member formed of a grounded electrically conductive member.
Furthermore, a method (hereinafter referred to as “method B”) for preventing charging unevenness due to the close placement of an upstream charging member and a downstream developing unit due to the miniaturization of image forming apparatus and close placement of image forming units is known. The apparatus includes cleaning means having a cleaning member and a grounded conductive support member (a shield member) that supports the cleaning member. The support member is located so as to shield the visual angle between the developing member of the developing means and the charging member that charges the image-bearing member next to the developing member.
However, it is difficult for method A to sufficiently achieve miniaturization because the shield member is located outward, away from the support member (casing) of the charging member. Moreover, to replace the charging unit, for example, it is necessary to remove the charging member together with the casing after the shield member is removed. This can affect the attachment/detachment of the charging unit complicating maintenance work.
Furthermore, due to the gap between the shield member and the casing, the space surrounded by the shield member can widen in the periphery of the portion between the charging unit to which the charging bias is applied and the image-bearing member. Thus, there is the possibility that an electric field, of the developing bias, can leak across the shield member (leaked electric field), which can affect the charging bias.
Furthermore, with method B, the support member (shield member) only covers a part of the casing of the cleaning member or the charging-roller cover (casing), so that the leaked electric field cannot be sufficiently prevented, which can affect the charging bias.